The present application relates to the modification of battery grids of the type used in lead-acid storage batteries, and more particularly, it relates to a modification of the surface finish of the battery grids of a lead-acid storage battery to improve paste adhesion and the service life of the battery.
Lead-acid storage batteries typically comprise several cell elements which are encased in separate compartments of a container containing sulfuric acid electrolyte. Each cell element includes at least one positive plate, at least one negative plate, and a porous separator positioned between each positive and negative plate. The positive and negative plates each comprise a lead or lead alloy grid that supports an electrochemically active material. The active material is a lead based material (i.e., PbO, PbO2, Pb or PbSO4 at different charge/discharge stages of the battery) that is pasted onto the grid. The grids provide an electrical contact between the positive and negative active materials which serves to conduct current.
Lead-acid battery manufacturing technologies and materials have improved in the last few decades. For example, because pure lead may be too soft for the manufacturing processes used to form battery grids, various alloying elements have been added to lead over the years to produce battery grids of sufficient strength to withstand battery manufacturing processes. For example, antimony was added to lead as lead-antimony alloys were found to be capable of being formed into battery grids at acceptable commercial rates by way of gravity casting techniques. However, it was discovered that when a lead antimony alloy is used in battery grids, water loss occurs because of gassing. Therefore, batteries having lead-antimony grids required periodic maintenance, i.e., the addition of water to the battery.
In order to lower the gassing rate of batteries, lead-calcium battery grids were developed. Batteries using lead-calcium alloy grids have relatively low gassing rates, and therefore, do not require the addition of water.
Another development in lead-acid battery manufacturing has been the manufacturing of battery plates in a continuous process, instead of traditional methods in which battery grids are made using a conventional gravity cast book mold operation and the cast grids are later pasted in a separate step. In a typical continuous battery plate making method, a lead alloy strip is manufactured, either by casting (namely, cast strip) or by casting and rolling (namely, wrought strip), and the strip is subsequently expanded or punched to generate the desired grid pattern in a strip of interconnected battery grids.
Typically, lead alloys having a relatively high level of calcium are used in continuous grid making processes as higher calcium levels tend to increase the hardness of the battery grids, which is beneficial in punching and expansion processes. Previously prepared active material battery paste (which may be prepared by mixing lead oxide, sulfuric acid, water, and optionally dry additives, such as fiber and expander) is then applied to the strip of interconnected battery grids and the strip is parted into single battery plates. Advantages of continuous battery plate making are improved production rate, improved dimensional control, thinner plates, lower scrap rate and lower manufacturing costs.
The pasted plates are next typically cured for many hours under elevated temperature and humidity to oxidize free lead (if any) and adjust the crystal structure of the plate. After curing, the plates are assembled into batteries and electrochemically formed by passage of current to convert the lead sulfate or basic lead sulfate(s) to lead dioxide (positive plates) or lead (negative plates). This is referred to as the “formation” process.
It is well known that lead-acid batteries will eventually fail in service through one or more of several failure modes. One such failure mode is failure due to corrosion of the grid surface. Electrochemical action corrodes the grid surface and reduces the adhesion between the active material and the grid (e.g., failure of the battery occurs when the grids are no longer able to provide adequate structural support or current flow due to the separation of the active material from the grid).
The formation efficiency of lead-acid batteries also may depend on the positive plate, in particular, to the extent of conversion of lead monoxide (PbO) to lead dioxide (PbO2) in the active positive material. The high electrical potential required for formation appears to be related to the transformation of non-conductive paste materials to PbO2. A low formation efficiency of positive plates requires a high formation charge. Inefficient charging also leads to deficiencies in the resulting batteries assembled with such plates. Typically, the initial capacity (performance) of the battery is low if the battery is not completely formed, requiring additional cycling to reach specific performance values. It is believed that by increasing the adhesion between the paste mixture and the grid, formation efficiency can be improved. Among other things, the increased adhesion between the grid and the paste provides for improved interfacial contact between the grid and paste thereby improving current flow between the grid and paste.
There is a need in the battery manufacturing field for even more effective methods for improving the service life of a battery. More particularly, there is a need for a method that can more greatly increase the adherence of active material to a battery grid produced by a continuous process.